when i switch on the circuit it works fine but after few seconds it goes dead ,
when i switch it off and switch on it immediately then also it's dead but i switch on after 5 - 10 seconds then it works finely but again same thing happens

i have attached a thick aluminum plate for heat sink

can the problem be because of slew rate of TL072 ic
(the slew rate is 13V/us ) or whether the circuit starts to oscillating.

Why R3? If you turn your bridge circuit on it causes half the supply voltage to go to the inverting amp. Tryit without - maybe lowering r4 to something lower. Does it work in normal (non bridged) mode? (my guess would be something is drifting to one of the supply voltages, then no signal goes through). Slew rate of op amp shouldn't cause problems in itself - worry about supply bypassing, layout and ground connections. Good luck.